Trisomy 21, Down syndrome (DS), affects approximately 400,000 people in the U.S., causing cognitive disability, which includes the neuropathology of Alzheimer's disease and late-life dementia. Based on the prevailing gene dosage effect hypothesis, a cognitively relevant phenotype in DS is caused by the triplication of one or more human chromosome (HSA) 21 genes. Our preliminary observations from the mouse-based studies suggest that these causative genes are indeed present and the search for them is both possible and productive. The long-term objective of this project is to identify these causative genes by using mouse-based genetic analysis, which is built upon the recent successes of our team: (1) We have developed two optimal reference mouse models for DS using efficient Cre/loxP-mediated chromosome engineering: Dp(16)1Yu/+, which is trisomic for the entire 22.9-Mb HSA21 syntenic region on mouse chromosome (MMU) 16, and Dp(10)1Yu/+;Dp(16)1Yu/+;Dp(17)1Yu/+, which is trisomic for all three HSA21 syntenic regions on MMU10, MMU16 and MMU17. (2) We have narrowed down the genomic region associated with the cognitive disability of DS to the smallest segment in the mouse genome: the Cbr1-Fam3b chromosomal segment containing 30 HSA21 gene orthologs. The triplication of this segment in mice causes abnormalities in cognitive behaviors, synaptic structures and hippocampal long-term potentiation, a major cellular mechanism that underlies learning and memory. To achieve our objective, we propose, in Specific Aim 1 of this application, to characterize the most important cognitively relevant phenotypes of the optimal reference mouse models for DS. To establish the basic phenotypic parameters to facilitate the genetic dissection, we will characterize the synaptic structures and plasticity in the hippocampus as well as cognitive behaviors of Dp(16)1Yu/+ and Dp(10)1Yu/+;Dp(16)1Yu/+;Dp(17)1Yu/+ mice. We will also measure the size and number of neurons in the hippocampal circuits of Dp(16)1/+ mice at the different ages to ascertain the neurodegenerative phenotype. In Specific Aim 2, we will analyze the Cbr1-Famb3b segment to identify a minimal genomic region for the DS- associated synaptic and cognitive phenotypes. We will generate new mouse mutants carrying nested duplications and deletions within the Cbr1-Fam3b segment by chromosome engineering and, by using these mutants, we will employ a subtractive/additive strategy in which synaptic and cognitive phenotypes are linked to progressively smaller genomic segments until a minimal critical region is defined. This effort will lay the groundwork to identify a causative gene(s) located within the minimal critical region(s) for these phenotypes, which will set the stage for the unraveling of the molecular mechanism of DS-associated cognitive disability as well as provide the conclusive support for the aforementioned hypothesis. Therefore, we expect, through these studies, to considerably accelerate progress in understanding and treating cognitive disability in DS.